Sight Triangle Calculator

At a stop-controlled intersection (the default control type), Major Road Leg is based on how long a stopped driver needs to accelerate across the major road's lanes -- and that required time gap gets SHORTER as Minor Road Speed increases, because a faster minor-road vehicle covers the intersection in less time. A shorter needed time gap means the major-road driver needs less advance warning distance, which is why Major Road Leg decreases as Minor Road Speed rises in this mode.

Grade and Perception-Reaction Time feed into the stopping sight distance (SSD) formula published in AASHTO's Green Book, which this calculator uses for yield-controlled and uncontrolled intersections. At a stop-controlled intersection, the governing calculation is instead a gap-acceptance time based on how long it takes a stopped vehicle to cross the lanes, which doesn't use the SSD formula at all, so Grade and Perception-Reaction Time have no effect on the result in that mode.

At an uncontrolled intersection, no sign assigns right-of-way to either approach, so both drivers need enough sight distance to see each other and stop if necessary -- unlike a stop-controlled intersection, where only the stopped driver needs to judge a safe gap. This calculator applies full stopping sight distance to both Major Road Leg and Minor Road Leg in that mode, using each road's own design speed.

Stopping Sight Distance (SSD) is the distance a vehicle needs to see a hazard and come to a complete stop, computed from Major Road Design Speed, grade, and perception-reaction time. Major Road Leg is the sight triangle dimension this calculator actually uses along the major road, which equals SSD directly for yield-controlled and uncontrolled intersections, but is calculated differently (from a gap-acceptance time) for stop-controlled intersections.